Methods for characterising and minimising the corrosive effects of an oil and oil plant making it possible to minimise the probelms ofcorrosion

ABSTRACT

The invention relates to a method of characterization of one petroleum, wherein the petroleum is placed in contact with a metal part, and which includes the following steps:
         i. Modifying the water content (W) of the petroleum,   ii. Measuring the corrosion (CR) of said metal part by the petroleum whose water content (W) has hence been modified,   iii. Building, by reiterating several times the two previous steps (i, ii), a database containing a plurality of different values of water content (W) of the petroleum and a plurality of values of corrosion (CR) each corresponding to one of said values of water content (W),   iv. Determining, by processing said database, an optimum value or an optimum range of values of water content (M W ) of the petroleum for which the corrosion of the metal part shows a minimum value (M CR ).       

     Problems of corrosion in the petroleum exploitation plants.

TECHNICAL FIELD

The present invention relates to the general field of petroleumexploitation and in particular the field of corrosion problems met insuch exploitation plants.

More particularly, the invention relates to a method of characterizationof one petroleum.

The invention also relates to a method of minimization of the corrosiveeffects of one petroleum.

The invention also relates to a petroleum exploitation plant comprisingat least one circuit formed at least in part of metal line elementswithin which petroleum circulates.

PRIOR ART

Petroleum exploitation requires to solve a certain number of technicalproblems, amongst which the corrosion of the petroleum exploitationplants by the petroleum. Indeed, petroleum, by its composition, provesto be intrinsically a corrosive medium. Moreover, this naturalcorrosivity is accentuated within petroleum exploitation plants by thehigh temperatures and flow velocities implemented. It hence resultstherefrom a premature degradation of the elements of such plants andparticularly of the inner side of the elements in which the petroleumcirculates, as for example lines, pipes, tanks, columns or other piecesof equipment, given that these latter are generally made of metalmaterial, a material that is not expensive and easy to implement butparticularly sensitive to the phenomenon of corrosion.

From then on, many problems, linked to the corrosion of such pieces ofequipment may appear within a petroleum exploitation plant, such as, forexample, leaks, cracks or fouling, causing in particular a lowering ofthe plant yield, a reduction of the flow rate, a waste of material, oreven environmental and industrial risks (explosions, fire, pollution,etc.), which can sometimes cause a total failure of the plant. Theseproblems being of course very harmful to the operators of a petroleumplant, many preventive and/or curative measures (maintenance,replacement, cleaning, etc.), on the different plant elements liable tobe impaired by the petroleum corrosivity, must be taken. It resultstherefrom an increase of the cost of exploitation of the petroleumexploitation plants.

These problems are particularly present in the plants treating crudepetroleums coming from deep petroleum reservoirs. Indeed, such “depth”petroleums are often particularly acid, and hence intrinsically verycorrosive. From then on, it is easy to understand all the stakes thatmay exist in finding efficient and cheap solutions to reduce thecorrosion problems that the petroleum plant operators are liable tomeet.

Different technical trails have been explored to develop solutionsmaking it possible to reduce the corrosion of the petroleum exploitationplants, and hence to try to overcome the above-mentioned problems.

A first solution consists in doing with the petroleum corrosivity andusing, to equip the petroleum exploitation plants, materials having ahigh corrosion resistance, as for example stainless or highly alloyedsteel. But the cost of such materials generally significantly reducesthe use thereof, so that it is often inconceivable to implement them ona large scale. These materials are hence the most often reserved inpractice to localized pieces of equipment of the petroleum exploitationplant.

A second alternative and/or complementary solution consists in trying toreduce the aggressiveness of the petroleum with respect to the pieces ofequipment of the petroleum exploitation plant. For that purpose, it iswidely known and used to inject into the petroleum different chemicalsubstances (mainly based on phosphorus and/or sulphur), called“corrosion inhibitors”. More precisely, these inhibitors, which areintrinsically corrosive components, will react to the contact with metaland then form a protective layer (iron phosphate or other) intended toprotect the metal wall from the effects linked to the petroleumcorrosivity . . . . This solution, only means currently available inpractice to reduce the corrosion phenomena, gives relatively satisfyingresults from the technical point of view, by contributing tosubstantially reduce the corrosion problems within petroleumexploitation plants. Although it asks no particular investment at theconstruction of the petroleum exploitation plant, this solution provesto be, on the other hand, expensive over the long term, taking intoaccount the cost of the chemical products implemented, and due to thefact that it is needed to permanently inject such products into thepetroleum so as to modify the surfaces of the plant elements (pieces ofequipment and lines, for example) to make them more resistant tocorrosion. This cost problem is all the more important in the plantsprocessing great quantities of petroleum since the quantities ofinhibitors (consumables) to be daily injected are proportional to theproduction rate of the units.

But, in addition to the substantial financial investment over the longterm, the petroleum so filled with inhibitors is liable to pose otherproblems to the petroleum plant operators. Indeed, the creation of aprotective layer (iron phosphate or other) by action of the corrosioninhibitors requires a highly active environment, so that the wholecorrosive effect is difficult to control and may often lead toundesirable and difficult to control collateral effects, as for example:

-   -   harmful effect of the inhibitors on the catalysts of the        downstream units, which degrades the efficiency of the global        process,    -   forming of a deposit inside the pieces of equipment, leading to        the fouling of the lines, exchangers or other pieces of        equipment.    -   etc.

Hence, from a global or systemic point of view, although the injectionof inhibitors into the petroleum potentially makes it possible tosubstantially reduce the corrosion of some parts of a petroleumexploitation plant (mainly the lines), other parts of the plant are onthe contrary degraded due to the collateral effects of the inhibitors,these latter being accentuated by parameters such that the pressureand/or the temperature (the activity of the high-temperature corrosioninhibitors evolves with the pressure and/or the temperature).

Eventually, the implementation of active agents such as the inhibitorsis paradoxically a non-negligible factor of risk for the good operationof the plant. This risk is aggravated by the fact that, in practice, aplurality of inhibitors are introduced in different and variableproportions, so that it is difficult, for the petroleum exploitationplant operation teams, to know and control the role of each of theinhibitors or chemical products injected into the petroleum, or simplyto identify a parameter influencing directly the corrosion. It thengenerally results therefrom, to determine the different dosages, the useof empirical methods, rather inaccurate and including a non-negligiblerisk of error.

DISCLOSURE OF THE INVENTION

The objects assigned to the present invention hence aim to remedy thedifferent drawbacks mentioned hereinabove and to propose a new method ofcharacterization of one petroleum, making it possible to identify, in asimple, fast, accurate and cheap manner, a parameter of minimization ofthe petroleum corrosion on a metal part.

Another object of the invention aims to propose a new method ofcharacterization of one petroleum, which is versatile and can beapplied, for example, to a great variety of different petroleums, and inparticular petroleums having very different total acid numbers (TAN).

Another object of the invention aims to propose a new method ofcharacterization of one petroleum, which can be implemented in aparticularly simple, safe, fast and cheap manner in a petroleumexploitation plant, including directly by the operator.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, which does notfundamentally modify the composition of the petroleum nor the degree ofacidity thereof.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, which isparticularly easy and simple to implement, while being particularlyaccurate.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, which is basedon an essentially analytical approach and not on a purely empiricalassessment.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, which requiresno heavy investment.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, constituting analternative to the use of chemical corrosion inhibitors based onphosphorus or sulphur and which is particularly interesting on theeconomic and environmental point of view.

Another object of the invention aims to propose a new method ofminimization of the corrosive effects of one petroleum, which can beimplemented within a petroleum exploitation plant in a particularlysimple, safe, fast and cheap manner, including directly by the operator.

Another object of the invention aims to propose a petroleum exploitationplant in which the problems of corrosion due to petroleum are minimized.

Another object of the invention aims to propose a petroleum exploitationplant that makes it possible to minimize, in a particularly simple andfast to implement manner, the problems of corrosion due to petroleum.

Another object of the invention aims to propose a petroleum exploitationplant that makes it possible to minimize, in a particularly economicalmanner, the problems of corrosion due to petroleum, withoutfundamentally impairing the composition and the properties of thepetroleum and by respecting the environment.

Another object of the invention aims to propose a petroleum exploitationplant that makes it possible to minimize, in an automatable manner, theproblems of corrosion due to petroleum.

Another object of the invention aims to propose a petroleum exploitationplant that makes it possible to minimize the problems of corrosion dueto different petroleums, whether they come from a single crude or fromdifferent cuts.

The objects assigned to the invention are achieved by means of a methodof characterization of one petroleum, wherein the petroleum is placed incontact with a metal part, and which includes the following steps:

-   -   i. Modifying the water content of the petroleum,    -   ii. Measuring the corrosion of said metal part by the petroleum        whose water content has hence been modified,    -   iii. Building, by reiterating several times the two previous        steps (i, ii), a database containing a plurality of different        values of water content of the petroleum and a plurality of        values of corrosion each corresponding to one of said values of        water content,    -   iv. Determining, by processing said database, an optimum value        or an optimum range of values of water content of the petroleum        for which the corrosion of the metal part has a minimum value.

The objects assigned to the invention are also achieved by means of amethod of minimization of the corrosive effects of one petroleum on ametal part, characterized in that the quantity of water in the petroleumis adjusted so that the water content is substantially equal to a valuefor which the corrosivity of said petroleum is minimum.

Finally, the objects assigned to the invention are also achieved bymeans of a petroleum exploitation plant comprising at least one circuitformed at least in part of metal line elements within which petroleumcirculates, characterized in that it comprises, on the one hand, a waterinjection device designed to inject water into said circuit so as tomodify the water content of the petroleum, in order to minimize thecorrosive effects of the petroleum on said metal lines, and on the otherhand, a means for determining the quantity of water to be injected.

BRIEF DESCRIPTION OF THE DRAWINGS

Other particularities and advantages of the invention will appear and bebetter understood from the reading of the following description, withreference to the appended drawings, given only by way of illustrativeand non-limitative example, in which:

FIG. 1 shows different curves of evolution of the rate of corrosion of ametal part (in mm/year) as a function of the quantity of water inpetroleum (in ppm) recorded at the time of execution of the methodaccording to the invention applied within the framework of a first andsecond particular examples.

FIG. 2 schematically shows a part of a refining plant within which thecorrosive effects of petroleum are minimized according to the invention.

FIG. 3 shows the curve of evolution of the water content delta as afunction of the initial water content recorded during a first particularexample of implementation of the invention.

FIG. 4 shows the curve of evolution of the water content delta as afunction of the initial water content recorded during a secondparticular example of implementation of the invention.

BEST WAY TO IMPLEMENT THE INVENTION

The invention relates as such to a method of characterization of onepetroleum, wherein the petroleum is placed in contact with a metal part.By characterization, it is meant any operation making it possible tocharacterize, i.e. to show, to indicate the phenomenon that occurs. Butthis term can also refer to any operation that permits to understand thephenomenon that is occurring. For example, in a non-limitative way, amethod of characterization generally comprises essential steps such astests, measurements or analysis of data.

Preferentially, the method object of the invention makes it possible tocharacterize the level of corrosion liable to be imparted to a piece ofequipment within which the petroleum in question circulates. Within theframework of the invention, the matter is preferentially the corrosionof a metal part that is in contact with one petroleum at hightemperature, i.e. for example at a temperature comprised between 200 and400° C., and preferentially of about 280° C. Indeed, by its composition,the petroleum is intrinsically a corrosive product. The term “corrosion”refers to the degradation of a part, or a material, by the action of asurrounding medium (herein petroleum), preferentially by a chemicalprocess. The corrosion has in particular for effect the loss of materialof the part or of the material in question, causing the thinning thereofand potentially going up to the perforation or even the destructionthereof.

The term “petroleum” refers both to the crude petroleum, i.e. petroleumas it comes from the petroleum reservoir during the drilling, and to thedifferent petroleum blends or to the different petroleum cuts that comefrom the petroleum distillation step that is generally performed in arefinery. Likewise, this term refers to a multitude of petroleums,whichever the geographical origins, maturities, extraction depth,viscosities, densities, or total acid number (TAN) thereof.Preferentially, the term “petroleum” refers to petroleums whose totalacid number, i.e. TAN, is different. The term “petroleum” may forexample refer to one petroleum from West Africa, whose natural totalacid number (TAN 1.5) may be artificially increased to TAN 5 (forexample, by addition of naphthenic acid into the crude) for the need ofsaid characterization.

The term “metal part” refers, without departing from the framework ofthe invention, to any part whose main composition is metal. Hence, thecomposition of this part may include any alloy and/or metal, alreadyknown or not, which includes at least one metal element, as for examplesteel, cast iron, stainless steel, etc. Preferentially, the term “metalpart” refers a steel part, and advantageously, a carbon steel of gradeASTM A105 representative, from the metallurgical point of view, of thecarbon steels frequently used in the petroleum industry. In this case,the phenomenon of corrosion of a metal part by one petroleum at hightemperature (a few hundreds of degrees Celsius) is in particular due tothe naphthenic acids of the petroleum that attack the carbon steel,hence forming a product of corrosion that is soluble in the organicphase, the iron naphthenate.

Similarly to its composition, the size and the shape of this part may beas varied as it can be imagined, without departing from the framework ofthe invention. Hence, the term “metal part” may refer both to a metalsheet of great size (a few meters) and to a plate of very small size (afew millimeters), for example to make as well a tank or a coupon,respectively. But this term may also refer to a part of more complexshape, such as a line, or a pipe, or even also an element, itselfconsisted of a set of parts, as for example a valve, a tap, or also adistillation column, for example within the framework of petroleumrefining.

Hence, taking into account the diversity of shapes, sizes and mattersthat may cover the term “metal part” without departing from theframework of the invention, the term “placed in contact” may hence referto a multitude of different situations, as long as the petroleum touchesdirectly said metal part. Hence, as an illustrative and non-limitativeway, the metal part may be in stationary contact, i.e. with no relativevelocity with respect to the petroleum, as for example in the case of atank, but may also in dynamic contact, i.e. a relative velocity existsbetween said metal part and the petroleum, as for example in the case ofa line or a pipe within which the petroleum circulates with a certainflow velocity, as it is the case in petroleum exploitation plants, or inpipelines. Preferentially, the metal part is placed in contact with thepetroleum with a relative velocity between the two materials comprisedbetween 0 and 15 m/s and preferentially between 1 and 3 m/s.

But this contact may also refer to an “artificial” contact, i.e. acontact reproduced for the needs of an experiment, in a laboratory,whether the latter belongs to a petroleum exploitation plant or not. Inthis case, the part placed in contact with the petroleum will bepreferentially a coupon, i.e. a metal part of a few square millimetersor centimetres and of simple shape, as for example a plate, aparallelepiped, a cylinder, a disk or a ring. Preferentially, thecoupons has the shape of disks of about 15 mm in thickness and 40 mm indiameter. In this case, in order to simulate a linear flow velocity,between the petroleum and the metal part, the coupon may be placed inrotation within the petroleum and hence provide a circumferentialvelocity representative of a linear velocity. Preferentially, saidcoupon may be placed in contact with the petroleum at a circumferentialvelocity of the order of 6 m/s, representative of the shear stresses inoperation.

The term “placed in contact” is not limited to a particular temperatureand covers a great diversity of temperatures at which the metal part isin contact with the petroleum, going in particular from the ambienttemperature to several hundreds of degrees Celsius. Advantageously, andas it is generally the case in the petroleum exploitation plants, themetal part is in contact with the petroleum at high temperature, i.e.for example at a temperature comprised between 200 and 400° C., andpreferentially of about 280° C.

Similarly, the term “placed in contact” is not limited to a particularpressure but covers a great diversity of pressures at which the metalpart is in contact with the petroleum, as for example a placement incontact under vacuum (null pressure), at the atmospheric pressure, or ata far higher pressure (several tens of bars, for example).

In the following description, reference will be made to a firstparticular example of implementation of the invention corresponding tothe curves C and D of FIG. 1, where the interest relates to thecharacterization of the corrosivity of one crude petroleum of WestAfrica, whose natural TAN is of about 1.5. The metal part used in thisexample is a coupon of carbon steel of grade ASTM A105, in the form of adisk of 40 mm in diameter and 15 mm in thickness. A multitude ofrigorously identical coupons is used for the good implementation of themethod, as will be described hereinafter. The coupon is mounted on anaxis and placed in rotation at contact with, i.e. inside, the WestAfrican petroleum, at a circumferential velocity of 6 m/s, substantiallyrepresentative of a linear flow velocity of 12 m/s in a line of 4inches. The temperature and the pressure at which this contact occursare of 280° C. and 70 bars, respectively, and that during 24 h. Ofcourse, this example is absolutely not limitative and the followingdescription is transposable, without exiting from the framework of theinvention, to another petroleum and/or another steel and/or otherconditions, etc.

According to the invention, said characterization method includes thefollowing steps:

-   -   i. A first step during which the water content W of the        petroleum is modified. Indeed, any crude petroleum intrinsically        includes a certain value of water at the time of the extraction        thereof from the petroleum reservoir. The characterization        method object of the invention proposes to modify this initial        water content W_(i) of the crude petroleum, either by        dewatering, or by addition of water, or possibly by successive        steps of dewatering then addition of water, and this, in order,        in a first time, to totally remove the water from the petroleum        so as to be able to very accurately control the water that is        added afterwards, hence starting from a basis where the value of        the water content W is substantially null. According to the        first particular example of implementation of the invention        described hereinabove, the water content of the West African        petroleum is varied by addition of water in its liquid form into        said petroleum. All the other intrinsic parameters of the        petroleum (and in particular the composition, viscosity,        density, maturity thereof) remain constant all over the process.    -   ii. A second step during which the corrosion CR of said metal        part by the petroleum, whose water content W has hence been        modified at the previous step is measured. Preferentially, the        corrosion measurement consists in determining a loss of material        of said metal part by time unit, as for example millimeters per        year. This matter loss measurement may advantageously be        performed by a mass loss measurement, in particular in the case        where the metal part is a coupon, and/or a thickness reduction        measurement, in particular in the case where the metal part has        a great size. Preferentially, it is also possible to perform a        pickling of the coupons (for example by means of an inhibited        acid) after their placement in contact with the petroleum, and        that, in order to determine the corrosion rate after exposure.        Indeed, the difference between the corrosion rate before and        after the coupon pickling gives information about a passive        and/or protective layer formed during the exposure, as will be        detailed hereinafter. According to the first particular example        of implementation of the invention described hereinabove, the        corrosion measurement is based on a calculation of matter loss        carried out in two steps: before the coupon pickling by acid        attack of the corrosion products and other different deposits of        the base metal, and after coupon pickling by acid attack, as        mentioned hereinabove.    -   iii. A third step during which is built, by reiteration several        times of the two previous steps (i, ii), a database containing a        plurality of different values of water content W of the        petroleum and a plurality of values of corrosion CR each        corresponding to one of said values of water content W, as        illustrated for example by Table 1.

TABLE 1 CR CR Measured W before after TAN Water pickling pickling (mgKOH/g) (ppm) (mm/year) (mm/year) 1.3 170 0.05 0.60 1.51 660 −0.07 0.131.5 1450 −0.01 0.17 1.52 4240 0.02 0.57 1.51 8690 0.15 0.59

-   -   -   The term “database” refers to any collection in which appear            different data items, and preferentially different            measurements. The database visible in Table 1 has been made            at the execution of the above-described first particular            example of implementation of the invention. According to            this example, the water content of the West African            petroleum is modified, so as to obtain the five different            values of water content W visible in Table 1. For each of            these values, the coupon corrosion measurements are hence            performed according to the above-mentioned protocol, so as            to observe the values of corrosion CR before pickling and            after pickling shown in the table hereinafter. Table 2 shows            a similar database that is constructed according to a second            particular example of implementation of the invention, whose            only difference with the first example described hereinabove            resides in the total acid number (TAN) of the petroleum            used, which has been artificially brought to 5, as mentioned            hereinabove.

TABLE 2 CR CR Measured W before after TAN Water pickling pickling (mgKOH/g) (ppm) (mm/year) (mm/year) 4.87 200 1.90 2.10 4.94 200 0.73 0.874.96 670 0.13 0.43 5.19 910 0.12 0.36 4.96 1490 0.89 1.07 5.16 1800 1.411.47 5.21 2580 0.20 0.30 5.12 3230 4.75 5.10 5.12 4130 1.72 2.43 5.18470 2.21 2.67 4.98 18740 0.44 1.57

-   -   iv. A fourth step during which it is determined, by processing        of said database, an optimum value or an optimum range of values        of water content M_(W) of the petroleum for which the corrosion        of the metal part shows a minimum value M_(DR). By “processing        of the database”, it is referred to any intervention that        consists in observing and/or processing and/or exploiting the        data coming from said database. The term “optimum range of        values” refers to a set of values (or one value in particular)        of water content M_(W) of the petroleum for which the corrosion        of the metal part shows a minimum value M_(CR). Preferentially,        the optimum range of values comprises several values about an        optimum value, so as to constitute a range of values, i.e. a        range or a zone of values. Advantageously, the database includes        enough different values of water content W of the petroleum and        of corresponding values of corrosion CR so as to be able to        observe, during the processing of said database, that, as the        water content W of the petroleum increases, the corrosion        progressively decreases then progressively increases. The term        “to observe” is herein understood within the meaning of        analysing, examining, the data to understand them and to be able        to draw conclusions. This observation is for example        advantageously realizable by means of the database visible in        the first table, coming from the first particular example of        implementation of the invention described hereinabove. From then        on, it can be clearly seen that, as the water content W of the        petroleum increases, the corrosion recorded on the metal part        tends to progressively reduce before progressively increase        again. More precisely, the observation of the database shows        that the corrosion value seems to decrease for the values of        water content of the petroleum of 170 to 1450 ppm, then seems to        increase beyond these values. A similar observation may be made        from the database shown in the second table, i.e. for the        petroleum having the TAN substantially equal to 5: the corrosion        reduces for the values of water content of the petroleum of 200        to 910 ppm, then seems to increase beyond these values.        -   Hence, a first observation or analysis of this database            makes it already possible to characterize, according to a            first approach, the level of corrosion of the petroleum on a            piece of equipment as a function of a parameter: the water            content thereof. It is hence already possible to determine a            first approached value, by a simple processing (for example,            an observation) of the database, of the optimum value of            water content of the petroleum M_(W) for which the corrosion            of the metal part shows a minimum value M_(CR). This            approached value would be visibly comprised between 660 and            1450 ppm of water in the case of the first example of            implementation of the invention, for the petroleum having a            TAN of 1.5 and between 910 and 1490 ppm of water of the            petroleum having a TAN of 5, in the case of the second            example. Nevertheless, it is delicate to determine with            accuracy the minimum value of corrosion M_(CR) and the            optimum value of water content M_(W) from the experimental            points presented hereinabove, so that it is preferable to            continue and/or to renew the processing of the database with            complementary operations described hereinafter.        -   Preferentially, said processing of the database comprises an            operation of plotting the curve representative of the            corrosion of the metal part as a function of the value of            the water content of the petroleum CR=f(W). In other words,            it is represented, on a diagram, the evolution of the metal            part corrosion as a function of the water content of the            petroleum value. The values of corrosion are placed on the            ordinate axis whereas the values of water content of the            petroleum are placed on the abscissa axis. Said curve            includes substantially two successive segments of curve as            the value of water content W of the petroleum increases:            -   a first segment of curve where the corrosion is a                decreasing function CR=f(W) of the water content W down                to a minimum point M, preferentially and substantially                according to the mode of variation of the inverse                function between 0 and 1,            -   a second segment of curve where the corrosion is an                increasing function CR=f(W) of the water content W from                the minimum point M, preferentially and substantially                according to the mode of variation of the logarithmic                function.        -   As an alternative, in this second segment of curve, the            corrosion is a substantially constant function CR=f(W) from            the minimum point M.        -   Preferentially, said curve further includes a third segment            of curve, where the corrosion tends to be stabilized            according to a substantially asymptotic tendency, in            particular for one petroleum having a relatively low TAN            (for example TAN of 1.5).        -   Such an operation is for example visible in FIG. 1. More            precisely, FIG. 1 includes four different curves coming from            the databases shown in the previous tables, showing on the            abscissa the water quantity W present in the petroleum (in            parts per million) and on the ordinate the corrosion CR of            the metal part (in matter loss rate, in millimeters per            year).            -   The curve A represents the corrosion of the metal part                after pickling, as a function of the water content of                the West African crude petroleum, according to the                second example of implementation of the invention                mentioned hereinabove.            -   The curve B represents the corrosion of the metal part                before pickling, as a function of the water content of                the West African crude petroleum, according to the                second example of implementation of the invention                mentioned hereinabove.            -   The curve C represents the corrosion of the metal part                after pickling, as a function of the water content of                the West African crude petroleum, according to the first                example of implementation of the invention mentioned                hereinabove.            -   The curve D represents the corrosion of the metal part                before pickling, as a function of the water content of                the West African crude petroleum, according to the first                example of implementation of the invention mentioned                hereinabove.        -   Each of the curves, although representing different            conditions, hence has substantially the same profile, i.e.            the same variations, as described hereinabove.        -   Advantageously, said processing of the database comprises            the search for the minimum point M, having for coordinates            on the abscissa said optimum value M_(W) of water content            and on the ordinate said minimum value M_(CR) of corrosion,            for which the derivative of the corrosion as a function of            the water content of the petroleum (dCR/dW) is substantially            null and where it passes from a negative sign to a positive            sign. In other words, if the table of variation of the            function CR=f(W) is constructed, it can be observed a            portion for with the derivative of said function is of            negative sign (which corresponds to a decreasing function)            then a portion for which the derivative is of positive sign            (which corresponds to an increasing function), the two parts            being separated by a value where the derivative of said            function is substantially null. This value then corresponds            to a minimum of said function, i.e. a value of water            quantity for which the corrosion is minimum. According to            the invention, this is precisely this value that is searched            to be identified, characterized. Indifferently, this search            for the minimum point M based on the derivative may be            performed in different manners, and in particular:            -   by literal differentiation, knowing the function CR=f(W)                and making the calculation of the derivative in a                literal manner, and/or            -   by a method of digital differentiation, i.e. by digital                processing of the database, visible for example on the                previous tables, et/or            -   by a method of graphic differentiation, using the                previously defined curve.        -   For example, the processing of the database shown in the            second table, and more particularly the analysis of the            curve A that follows therefrom, plotted in FIG. 1, shows            that the minimum point M has for coordinates, on the            abscissa, the approximate value of 1000 ppm of water and on            the ordinate the approximate value of 0.31 mm/year of            corrosion. In other words, in this second particular example            of implementation of the invention, the water quantity M_(W)            for which the corrosivity of this petroleum is minimum            M_(CR) is of about 1000 ppm.        -   Said processing also shows, in the first and second            particular examples of implementation of the invention, that            this value M_(W) is substantially common for the different            curves A, B, C, D, which means that said value M_(W) is            herein independent of the total acid number (TAN) of the            petroleum. It is however perfectly conceivable that, for            other petroleums, the values given hereinabove are            substantially or totally different, and that without            departing from the framework of the invention.

According to a preferential embodiment of said method ofcharacterization, said metal part is consisted of a metal line insidewhich the petroleum circulates, said line belonging to a petroleumexploitation plant, as described hereinabove (the term “petroleumexploitation plant” will be defined hereinafter). For example, theinterest relates to the characterization of the corrosivity of onepetroleum circulating within a petroleum exploitation plant, for examplea refinery, and more precisely circulating inside a metal line.According to this embodiment, the water content W of said petroleum willbe modified, then the corrosion CR of the metal line will be measured,after a certain period of time (for example, several days or weeks) thatis substantial enough to record a corrosion of the line. Afterwards, thesteps of modification of the water content W of the petroleum and ofmeasurement of corrosion CR are reiterated, still with the same periodof time, so as to construct a database. Then, thanks to the processingof said database, it will be possible to determine the optimum value ofwater content W_(C) of the petroleum for which the corrosion of themetal line shows a minimum value M_(CR).

The characterization method described hereinabove hence makes itpossible to highlight, in particular thanks to the processing of thedatabases, that a very clear relation exists between the water contentand the petroleum corrosivity, and that for different levels of totalacid numbers (as it has been shown for the West African petroleum of TAN1.5 and TAN 5 during the examples of implementation of the inventiondescribed hereinabove). In other words, the water content of onepetroleum is clearly a determining factor to determine the intrinsiccorrosivity of one petroleum and/or the corrosive effects thereof.

The hereinabove characterization method also makes it possible tohighlight that other essential parameters come into play in thecharacterization of the corrosion of one petroleum. Amongst theseparameters, some of them are intrinsic to the petroleum and other onesare linked to the process implemented. Hence, in addition to the watercontent of the petroleum, the total acid number of one petroleumrepresented by the value of the TAN has a predominant effect on thecorrosivity thereof. As illustrated for example on the curves visible inFIG. 1, other things being equal, i.e. all the other parameters beingidentical (temperature, pressure, velocity, etc.), the more the TAN ofone petroleum is high, the more the corrosivity thereof, i.e. thecorrosion potential and hence the corrosive effects thereof will behigh, and hence, the more the injection of water will have an importantrole to play in the method of minimization that is describedhereinafter. The relative velocity between the petroleum and the metalpart, in other words the flow velocity of the petroleum, is also anessential parameter in the characterization of the corrosivity of onepetroleum. Indeed, the more the flow velocity is important, the more theaggressiveness of the petroleum will be high, by combination of thephenomena of erosion and corrosion.

Hence, amongst the different parameters mentioned hereinabove (TAN,temperature, pressure, velocity, water content of the petroleum, etc.)that all have a direct influence on the petroleum aggressiveness withrespect to the plants, the invention has highlighted the major andunexpected role of the water content W of the petroleum, this parameterfurther having the advantage to be easily modifiable, without affectingthe other parameters and without generating collateral effects liable toharm the exploitation.

The invention also relates, as such, to a method of minimization of thecorrosive effects of one petroleum on a metal part. By “minimizationmethod” it is meant any method by which it will be searched tosubstantially lower, reduce, minimize the corrosive effects and/or thecorrosivity of the petroleum. The terms “corrosive”, “petroleum”, “metalpart” are defined in the same way as hereinabove.

According to the invention, said minimization method consists inparticular in adjusting the quantity of water in the petroleum so thatthe water content W of the petroleum is substantially equal to a valuefor which the corrosivity of said petroleum is minimum. The verb “toadjust” herein refers to any operation that consists in adapting, i.e.tuning, making correspond the water quantity W of the petroleum to avalue close to the value for which the petroleum corrosivity is minimum.Preferentially, said quantity of water in the petroleum can be adjustedby addition of water. Furthermore, the addition of water may becompleted or replaced by a removal of water, for example by means of adewatering of the petroleum. This step may in particular make itpossible, by being performed previously to the addition of water, torealize the injection of water from one petroleum that will have beentotally dewatered, i.e. deprived of its initial internal water, so thatthe quantity of water of the petroleum will be substantially equal tothe quantity of water that is added into said petroleum. Hence, it won'tnecessarily be required to measure the quantity of water of thepetroleum, given that an accurate estimation will be directly deducedfrom the quantity of water added. Advantageously, the addition of wateris performed in its liquid and/or gaseous form, i.e. in the form ofwater vapour, and preferentially from demineralized water.

Advantageously, said water content value W corresponds to the optimumvalue of water content M_(W) determined by means of the characterizationmethod described hereinabove.

Preferentially, the quantity of water in the petroleum is adjusted sothat the water content of the petroleum is substantially comprisedbetween 200 and 2000 ppm, advantageously between 500 and 1500 ppm, andpreferentially of the order of 1000 ppm, these values havingadvantageously been determined following the implementation of thecharacterization method presented hereinabove, for example according tothe first or second example of implementation of the invention describedhereinabove, implementing West African petroleum. These values arefurthermore low enough (of the order of 0.1% in mass) so as not tofundamentally modify neither the petroleum composition nor the behaviourthereof during the process, and to represent a minor, or evennegligible, impact on the operational, economic plan, and asubstantially null impact on the environmental plan.

Furthermore, and as can be seen on the curve A of FIG. 1, the control,i.e. the adjustment or the regulation, of the water content W of thepetroleum makes it possible to reduce to substantially 90% theconsumption of matter on a metal part, linked to the corrosion of thelatter under the effect of the petroleum corrosivity.

According to a preferential embodiment of said minimization method, saidmetal part is consisted by a metal line inside which petroleumcirculates, said line belonging to a petroleum exploitation plant, aspreviously described (the term “petroleum exploitation plant” is definedhereinafter). In other words, it comes to implement the method ofminimization of the corrosive effects, in conditions similar to thosedescribed hereinabove in the previous embodiment of the characterizationmethod, where the quantity of water will then be adjusted so as to giveto the petroleum substantially a water amount value for which thecorrosivity of said petroleum is minimum.

More precisely, the developed method of minimization of the petroleumcorrosivity has for objective to create a protective layer on the metalpart, which will then protect the latter from the attacks of thenaphthenic acids of the petroleum at high temperature. This protectivelayer may include magnetite (Fe₃O₄), which may provide a protectivecharacter by itself, and/or in synergy with iron sulphur (FeS), inaccordance with the following fundamental equations:

3Fe+4H₂O

Fe₃O₄+4H₂

and/or

2R-SH+4H₂O+4Fe

Fe₃O₄+FeS+R₂-S+5H₂

Indeed, the layer of iron sulphur is the direct result of the hot attackby sulphur, whether the latter is intrinsically contained in thepetroleum or, in a lesser extent, possibly added in the form ofinhibitors. But, as shown by the above equations, the injection of water(H₂O) further allows the formation of magnetite, which has for effect toprotect against the naphthenic acids of the petroleum at hightemperature by itself and/or by stabilization and reinforcement of thelayer of iron sulphur, hence increasing the corrosion resistance of themetal part. More precisely, the stabilizing effect of the magnetite onthe layer of iron sulphur makes it possible to increase the resistanceto the effect of dissolution of the iron sulphur by the naphthenicacids.

Nevertheless, the methods developed within the framework of theinvention have also highlighted that this layer of magnetite isrelatively difficult to obtain and/or to maintain, and that the quantityof water to be adjusted should be located in a range of values where thequantity of water is relatively low to reach a particularly low level ofcorrosion. Indeed, as illustrated by the previous data, and inparticular the curves visible in FIG. 1, if the quantity of waterbecomes too high, for example beyond 2000 ppm for the West Africanpetroleum, the corrosion rate of the metal part increases again to reachthe value initially measured with the petroleum before water is injectedinto it, or even to exceed this level. Indeed, the methods ofminimization and characterization implemented have also made possible tohighlight that, beyond the optimum water content M_(W), the water in toogreat quantity seems to contribute to create an acid environment, inparticular by dissociation of the organic acids in the water. From thenon, the development of this acid environment would tend to inhibit theaction of protection of the magnetite, in particular by leading to itsdissolution according to the following reaction:

Fe₃O₄+8H⁺→3Fe²⁺+4H₂O

As a consequence, it is crucial to correctly and accurately identify theoptimum point of water content M_(W) for which the corrosion is minimum,as described hereinabove, otherwise there exists a real risk to getagain the initial corrosivity of the petroleum, or even to increase it,and hence, to aggravate the problems of corrosion of the plants.

Another remarkable phenomenon has been highlighted during theelaboration of these methods of characterization and minimization of thecorrosivity of one petroleum and directly relates to the injected water.Indeed, as illustrated by FIGS. 3 and 4, the injected water can, as afunction of the total acid number (TAN) of the petroleum, be totallyconsummated or, on the contrary, increased, then meaning that water hasbeen created during the method. FIG. 3 relates to the first example ofimplementation of the invention mentioned hereinabove and represents thecurve R of evolution of the water delta (in ppm), i.e. the differencebetween the initial quantity of water (i.e. the quantity of waterinjected into the petroleum) and the final quantity of water (i.e. thequantity of water present in the petroleum once the corrosionmeasurement terminated) as a function of the initial quantity of water.FIG. 4 shows in a same way the curve F according to the second exampleof implementation of the invention mentioned hereinabove. FIG. 3 showsthat, up to a certain quantity of water injected in the petroleum(approximately 7000 ppm), the final quantity of water (i.e. the quantityof water measured after the corrosion measurement after 24 h, asmentioned hereinabove) is higher than the initially injected water. Inother words, water has been produced during this test. Likewise, FIG. 4shows that water is also produced, but this time up to an initial valueof water quantity of approximately 2000 ppm. This production/consumptionof water has very likely a relation with the stability of the previouslydescribed protection layer that is created on the steel, even if thequantities of water created or consumed indicate that other chemicalreactions, between other components present in the petroleum, come intoplay.

The invention also relates as such to a petroleum exploitation plant.This term refers to any plant intended for the exploitation ofpetroleum, going from a drilling plant, such as a petroleum platform ora drilling well, to a refining plant, such as a refinery. By extension,and without departing from the framework of the invention, any plantallowing in a more or less direct manner petroleum exploitation, isconcerned by this expression, as for example a pipeline or a tank.

FIG. 2 schematically and illustratively shows the principle of operationof a petroleum exploitation plant, a refinery, whose object is totransform the crude petroleum into a multitude of finished and/orsemi-finished products (such as fuels, bitumens, lubricants . . . ). Ina known manner, the operation of such a plant begins with a source ofcrude petroleum 1 that comes, for example, from a tank or directly froma pipeline. The crude petroleum is, in a first time, conveyed by meansof lines 12 in a desalter 2, which includes a water intake 21 and adevice for collecting water and mineral salts 22. The main function ofthis desalter is to extract the different mineral salts that may containthe petroleum, as for example the sodium, magnesium and calciumchlorides, so as to avoid the hydrolysis of these salts and theformation of very aggressive compounds (for example hydrogen chloride)and, in a lesser extent, that these salts are deposited, in particularunder the effect of heat, in the different elements of the plant. In aknown manner, this desalting function is, for example, made by injectinga great quantity (about 5%) of soft water, hence forming an emulsion,which, under the action of an electrostatic field will favour theagglomeration of the water drops containing the mineral salts, whichwill then be extracted after settlement 22. Once “desalted”, thepetroleum is then conveyed by means of lines 23 to a furnace 3 so as toheat the petroleum to a high temperature. Then the petroleum is conveyedtowards the distillation column (or tower) 4. A distillation columnincludes different draw-off trays 41, 42, 43, and is intended toseparate the molecules as a function of their molecular masses toproduce different petroleum cuts. These petroleum cuts are thenextracted in different draw-off circuits 611, 621, 631. By way ofillustration, the circuit 61 is for example a circuit making it possibleto obtain domestic fuel oil, the circuit 62, gasoil, the circuit 63,gasoline, etc. The circuit 5, located at the bottom of the column, makesit possible to obtain, for example, heavy residuals such as bitumen,whereas the circuit 7, located right at the top of the column, makes itpossible to obtain gases such as butane or propane. The differentdraw-off circuits may include different more or less complex elements soas to produce the different finished or semi-finished products mentionedhereinabove. Of course, the elements mentioned hereinabove are given byway of illustrative and non-limitative example, and their number hasbeen reduced in the Figure for the sake of clarity. Indeed, in practice,a distillation column includes more elements, and in particular a moreimportant number of draw-off circuits.

According to the invention, said petroleum exploitation plant comprisesat least one circuit formed at least in part of metal line elementswithin which petroleum circulates. By “lines”, it is understood anyelement intended to convey the petroleum from a first point to a secondpoint, as for example a pipe or a pipeline. By way of illustration, theline elements may be consisted by all or part of the lines 12, 23, 611,621, 631, visible in FIG. 2. The term “metal” is understood in the sameway as described above. The so-defined line elements are organized as acircuit, i.e. they form either a closed loop or an open loop so as totransport the petroleum from a starting point to an arrival point,wherein these latter can be different (in the case of an open circuit)or merged together (in the case of a closed circuit).

According to the invention, the petroleum exploitation plant comprises,on the one hand, a water injection device 8. The term “water injectiondevice”, refers to any device, means, element, member or set of elementswhose main function is to inject water into another element. In otherwords, the water injection device 8 is able to let the water passthrough from a first medium to a second medium but not reciprocally.Preferentially, said water injection device operates intermittently,i.e. it is capable of letting the water pass in one direction at certainmoments, but also of being totally tight at other moments. Said device 8may, for example, by way of illustrative and non-limitative example, beconsisted by a tap or a valve equipped with a check valve, so that thewater or the liquid can circulate only in one direction. Preferentially,said tap or said valve can be operated from a totally closed and tightstate to a totally open state, by passing through a multitude ofintermediate positions, in a continuous or on the contrary discretemanner, so as to make it possible to adjust the flow rate, i.e. toregulate the quantity of water to be injected. Advantageously, saiddevice is able to be remote-controlled. Naturally, said water injectiondevice is chosen amongst the known elements (valve, tap, nozzle, etc.)or made in such a manner to support the pressure and temperaturestresses linked to a petroleum exploitation plant.

According to the invention, said water injection device 8 is designed toinject water into said circuit so as to modify the water content W ofthe petroleum that circulates in the circuit, in order to minimize thecorrosive effects of the petroleum on said metal lines, as explainedhereinabove.

According to the invention, the petroleum exploitation plant includes,on the other hand, a means for determining the quantity of water to beinjected. By “means”, it is understood any device or method that makesit possible to know the quantity of water to be injected, in otherwords, that makes it possible to define a target quantity of water to bereached by injection of water. Preferentially, said means to determinethe quantity of water to be injected implements a method ofcharacterization according to the previous description, so as todetermine the optimum water content M_(W) for which the corrosion CR isminimum. But, alternatively or complementarily, the means fordetermining the quantity of water to be injected may also be consistedof a collection of graphical charts, which, as a function of theintrinsic properties of the petroleum and/or of the conditions of use(flow velocity, temperature, line materials, etc.), would give theoptimum quantity of water that the petroleum must contain to minimizethe corrosion. In other words, it would be perfectly conceivable,without departing from the framework of the invention, to use, as adetermination means, the existing curves visible in FIG. 1, whichrepresent by themselves a graphical chart for the West African petroleumin the conditions of use that are described hereinabove. Advantageously,said means for determining the quantity of water to be injected isconsisted by a database comprising different characteristics of thedifferent petroleums used in the plant, so as to allow an automaticrecognition of the petroleum used and an automatic determination of thequantity of water to be injected, as a function of said petroleum, tominimize the corrosive effects of the petroleum. Preferentially, saiddatabase is fed by a self-learning process which, as the exploitation ofthe petroleum plant goes along, records preferentially automatically thedifferent parameters and other measured and/or calculatedcharacteristics, linked to the different petroleums used.

Preferentially, said circuit comprises a distillation column 4 thatitself includes at least one draw-off circuit 61 (as illustrated by theabove-described FIG. 2), the water injection device 8 being connected tosaid draw-off circuit 61 so as to inject water into the latter. The term“connected” herein refers to any type of tight connection between thewater injection device 8 and the draw-off circuit 61, as for example aconnection made by means of an operation of cutting said circuit 61followed by an operation of welding said water injection device 8 on thecircuit. But any other assembly, as for example an assembly by means ofgaskets, screws and nuts, may perfectly be implemented without departingfrom the framework of the invention. Preferentially, said waterinjection device 8 is connected to the draw-off circuit 61 by beingarranged at the nearest position to the distillation column 4, and thatin order to protect the whole draw-off circuit 61.

Preferentially, the water injection device 8 is designed to inject waterin the form of vapour, and is hence consisted, for example, by aninjection nozzle. Indeed, the injection of water in its gaseous form(vapour) allows the formation of the magnetite, hence improving theprotection of the lines according to the principle detailed hereinabove.Furthermore, the vapour has the advantage to be easy to distribute andis immediately miscible, by vaporization inside the medium circulatingin the previously defined circuit, i.e. inside the petroleum. Finally,great quantities of vapour are generally available in a petroleumexploitation plant, in the form of medium pressure steam (of the orderof 10 to 20 bars), being hence directly usable by the water injectiondevice 8. Advantageously, said water injection device makes it possible,using the vapour already present in the plant, to regulate the vapourfrom a general point of view.

Advantageously, the petroleum exploitation plant includes an evaluationdevice 9 for evaluating the level of corrosion CR of at least one ofsaid line metal elements of the circuit in which the petroleumcirculates. The term “evaluation device” refers to any device, i.e. anymeans making it possible to estimate and/or measure the corrosion. Sucha device may, for example, be made by a measurement of thickness of themetal element in question, due to the fact that the corrosion is aphenomenon that reduces the thickness of the metal element.Advantageously, such a device may be made in a non-destructive andcontinuous manner, for example by means of a corrosion rate measurementprobe, or a corrosion control device. Preferentially, such a probe is inthe form of a device fixed to the line and that emits ultra-sound wavesthat will pass through the wall of the line. The refraction of thesewaves, within different materials constituting the line will then makeit possible to give an indication, following operations of signalprocessing, in a first time of the thickness of the line, and in asecond time of the rate or level of corrosion of the line. But thecorrosion evaluation device may also be made by means of any otherdevice, as, for example, a visual device (camera, endoscope, etc.) orchemical device (making it possible to detect the presence and thequantity of corrosion products) from the moment that the latter makes itpossible to obtain an evaluation of the level of corrosion of at leastone of the line metal elements of the circuit within which the petroleumcirculates.

Preferentially, said corrosion level evaluation device 9 is connected tosaid draw-off circuit 61 mentioned hereinabove and visible in FIG. 2(the term “connected” being herein defined in the same as previously),preferably after the water injection device 8, said water injectiondevice 8 being controlled based on information coming from saidevaluation device 9, through a processing means. The term “information”herein refers to any type of data, as for example digital or analogdata, coming preferentially from measurements made by said device. By“processing means” it is understood any device able to determine, basedon the above-described information, the way to control the waterinjection device, i.e. to give it the quantity of water to be injected.Such a means may be for example consisted by a computing terminal of thecomputer type, which is able to determine automatically the quantity ofwater to be injected as a function the information coming from theevaluation device 9, preferentially by implementing in an automatedmanner the methods of characterization and/or minimization definedhereinabove. But, alternatively or complementarily, the processing meansmay also be consisted of a collection of graphical charts, which,according to the level of corrosion and/or to the intrinsic propertiesof the petroleum and/or to the conditions of use (flow velocity,temperature, materials of the lines, etc.) would give the optimumquantity of water that the petroleum must contain to minimize thecorrosion.

Advantageously, the petroleum plant includes a device for evaluating thewater content W of the crude petroleum and/or of the cuts thereof and/orof the blends thereof, said water injection device 8 being controlledbased on information coming from said device for evaluating the watercontent W of the petroleum through a processing means. In other words,instead of, or in addition to, controlling the water injection device 8as a function of the corrosion level as defined hereinabove, it is alsopossible to control the latter using a means that makes it possible toprovide the quantity of water contained by the petroleum. In otherwords, this device supposes to know beforehand, as a function of theconditions of the plant (materials, characteristics of the petroleum,temperature, velocity . . . ), the theoretical optimum quantity of waterthat the petroleum must contain, so as to minimize the corrosion. Forexample, following the previous description, it is known, that theoptimum value of water content M_(W) of the West African petroleum, tominimize the corrosivity (and/or the corrosive effects) thereof issubstantially of 1000 ppm. It is hence possible, simply by means of thewater content evaluation device, to control the water injection device8, so as to reach the optimum value M_(W). Furthermore, the presence ofa device for evaluating the water content W also makes it possible tomake a closed-loop control system, making it possible to maintainautomatically the water content of the petroleum close to the optimumvalue. Hence, the previous descriptions relative to the devices,information elements and processing means also apply to this watercontent evaluation device.

The use of one or several of the previous evaluation devices (evaluationof the level of corrosion and/or of the water content of the petroleum)further makes it possible to automatize the injection of water into thepetroleum, i.e. the plant is able, by itself, with no human interventionor with a minimum of human intervention, to adjust the quantity of waterin the petroleum so as to minimize the corrosion on the differentelements of said plant, and this, advantageously, substantially in realtime. Hence, even if one of the previously mentioned parameters shouldevolve (velocity, petroleum change, temperature, pressure, etc.), thelevel of corrosion and/or the water content of the petroleum wouldevolve, the means for controlling would detect it and would then be ableto automatically modify the instruction sent to the water injectiondevice so as to modify, if need be, the water content of the petroleum.

Advantageously, the plant includes a multitude of water injectiondevices and of corrosion level evaluation devices 9 and/or water contentof the petroleum evaluation devices, associated by pair (i.e. at leastone evaluation device associated with an injection device 8) indifferent circuits, so that each pair of devices operates independentlyfrom each other.

Advantageously, one pair of these devices equips each of the draw-offcircuits of the distillation column, which makes it possible to adjustthe quantity of water to be injected in each of the circuits as afunction of the proper characteristics of each of the petroleum cuts.

POSSIBILITY OF INDUSTRIAL APPLICATION

The invention finds an industrial application in particular in theimplementation of a method of characterization of one petroleum and/orof a method of minimization of the corrosive effects of one petroleum ina petroleum exploitation plant, and/or in the design, making andexploitation of a petroleum exploitation plant comprising at least onecircuit formed in part of metal line elements inside which petroleumcirculates, as for example a petroleum platform or a refinery.

1- A method of characterization of one petroleum, wherein the petroleumis placed in contact with a metal part, and which includes the followingsteps: i. Modifying the water content (W) of the petroleum, ii.Measuring the corrosion (CR) of said metal part by the petroleum whosewater content (W) has hence been modified, iii. Building, by reiteratingseveral times the two previous steps (i, ii), a database containing aplurality of different values of water content (W) of the petroleum anda plurality of values of corrosion (CR) each corresponding to one ofsaid values of water content (W), iv. Determining, by processing saiddatabase, an optimum value or an optimum range of values of watercontent (M_(W)) of the petroleum for which the corrosion of the metalpart shows a minimum value (M_(CR)). 2- The method of characterizationof one petroleum according to claim 1, characterized in that thedatabase includes enough different values of water content (W) of thepetroleum and of corresponding values of corrosion (CR) so as to be ableto observe, during the processing of said database, that, as the watercontent (W) of the petroleum increases, the corrosion progressivelydecreases then progressively increases. 3- The method ofcharacterization of one petroleum according to claim 1, characterized inthat said processing of the database comprises an operation of plottingthe curve representative of the corrosion of the metal part as afunction of the value of the water content of the petroleum (CR=f(W)),said curve including substantially two successive segments of curve asthe value of water content (W) of the petroleum increases: a firstsegment of curve where the corrosion is a decreasing function (CR=f(W))of the water content (W) down to a minimum point (M), a second segmentof curve where the corrosion is an increasing function (CR=f(W)) of thewater content from the minimum point (M). 4- The method ofcharacterization of one petroleum according to claim 1, characterized inthat said processing of the database comprises the search for theminimum point (M), having for coordinates on the abscissa said optimumvalue (M_(W)) of water content and on the ordinate said minimum value(M_(CR)) of corrosion, for which the derivative of the corrosion as afunction of the water content of the petroleum (dCR/dW) is substantiallynull and where it passes from a negative sign to a positive sign. 5- Themethod of characterization of one petroleum claim 1, characterized inthat said metal part is consisted by a metal line inside which petroleumcirculates, said line belonging to a petroleum exploitation plant. 6- Amethod of minimization of the corrosive effects of one petroleum on ametal part, characterized in that the quantity of water in the petroleumis adjusted so that the water content (W) of the petroleum issubstantially equal to a value for which the corrosivity of saidpetroleum is minimum. 7- The method of minimization of the corrosiveeffects of one petroleum according to claim 6, characterized in thatsaid value of water content (W) corresponds to the optimum value (M_(W))of water content determined by means of the method according to theobject of any one of claims 1 to
 5. 8- The method of minimization of thecorrosive effects of one petroleum according to claim 6, characterizedin that the quantity of water in the petroleum is adjusted so that thewater content of the petroleum is substantially comprised between 200and 2000 ppm, advantageously between 500 and 1500 ppm, andpreferentially of the order of 1000 ppm. 9- The method of minimizationof the corrosive effects of one petroleum according to claim 6,characterized in that the quantity of water in the petroleum is adjustedby addition of water. 10- The method of minimization of the corrosiveeffects of one petroleum according to claim 6, characterized in thatsaid metal part is consisted by a metal line inside which petroleumcirculates, said line belonging to a petroleum exploitation plant. 11- Apetroleum exploitation plant comprising at least one circuit formed atleast in part of metal line elements within which petroleum circulates,characterized in that it comprises on the one hand a water injectiondevice (8) designed to inject water into said circuit so as to modifythe water content (W) of the petroleum in order to minimize thecorrosive effects of the petroleum on said metal lines, and on the otherhand a means for determining the quantity of water to be injected. 12-(canceled) 13- The petroleum exploitation plant according to claim 11,characterized in that said circuit comprises a distillation column (4)that itself includes at least one draw-off circuit (61), the waterinjection device (8) being connected to said draw-off circuit (61) so asto inject water into the latter. 14- The petroleum exploitation plantaccording to claim 11, characterized in that the water injection device(8) is designed to inject water in the form of vapour. 15- The petroleumexploitation plant according to claim 11, characterized in that itincludes an evaluation device (9) for evaluating the level of corrosion(CR) of at least one of said line metal elements of the circuit in whichthe petroleum circulates. 16- The petroleum exploitation plant accordingto claim 13, characterized in that said evaluation device (9) isconnected to said draw-off circuit (61), said water injection device (8)being controlled based on information coming from said evaluation device(9), through a processing means. 17- The petroleum exploitation plantaccording to claim 11, characterized in that it includes a device forevaluating the water content (W) of the crude petroleum and/or of thecuts thereof and/or of the blends thereof, said water injection device(8) being controlled based on information coming from said device forevaluating the water content (W) of the petroleum through a processingmeans. 18- A petroleum exploitation plant comprising at least onecircuit formed at least in part of metal line elements within whichpetroleum circulates, characterized in that it comprises on the one handa water injection device (8) designed to inject water into said circuitso as to modify the water content (W) of the petroleum in order tominimize the corrosive effects of the petroleum on said metal lines, andon the other hand a means for determining the quantity of water to beinjected, wherein determining water quality of water to be injectedimplements a method according to claim 1.